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191.
Limited occurrence of denitrification in four shallow aquifers in agricultural areas of the United States 总被引:1,自引:0,他引:1
Green CT Puckett LJ Böhlke JK Bekins BA Phillips SP Kauffman LJ Denver JM Johnson HM 《Journal of environmental quality》2008,37(3):994-1009
The ability of natural attenuation to mitigate agricultural nitrate contamination in recharging aquifers was investigated in four important agricultural settings in the United States. The study used laboratory analyses, field measurements, and flow and transport modeling for monitoring well transects (0.5 to 2.5 km in length) in the San Joaquin watershed, California, the Elkhorn watershed, Nebraska, the Yakima watershed, Washington, and the Chester watershed, Maryland. Ground water analyses included major ion chemistry, dissolved gases, nitrogen and oxygen stable isotopes, and estimates of recharge date. Sediment analyses included potential electron donors and stable nitrogen and carbon isotopes. Within each site and among aquifer-based medians, dissolved oxygen decreases with ground water age, and excess N(2) from denitrification increases with age. Stable isotopes and excess N(2) imply minimal denitrifying activity at the Maryland and Washington sites, partial denitrification at the California site, and total denitrification across portions of the Nebraska site. At all sites, recharging electron donor concentrations are not sufficient to account for the losses of dissolved oxygen and nitrate, implying that relict, solid phase electron donors drive redox reactions. Zero-order rates of denitrification range from 0 to 0.14 micromol N L(-1)d(-1), comparable to observations of other studies using the same methods. Many values reported in the literature are, however, orders of magnitude higher, which is attributed to a combination of method limitations and bias for selection of sites with rapid denitrification. In the shallow aquifers below these agricultural fields, denitrification is limited in extent and will require residence times of decades or longer to mitigate modern nitrate contamination. 相似文献
192.
Highly efficient regeneration of natural zeolite was developed in conjunction with the removal of high concentrations of ammonia and potassium from the reverse osmosis effluent of anaerobic-digested wastewater by fixed-bed ion exchange.The elution and uptake behavior of ammonium and potassium in the fixed bed were studied.Both batch desorption tests and on-column regeneration were conducted to develop an optimum regeneration condition compatible with the wastewater requirements.The effectiveness of ammonium elution increased with increasing alkaline concentration.The increase of salt dose significantly enhanced the ammonium maximum in the elution solution.Complete ammonium elution was achieved in 6 bed volumes(BV) when the alkaline and salt concentrations were respectively 0.1 mol/L and 18.6 g/L at a flow rate of 2.5-3.0 BV/hr.Due to the higher affinity of potassium with natural clinoptilolite,complete potassium elution was not achieved in all cases. 相似文献
193.
Zheng Li Andreas Sjodin Erin N. Porter Donald G. Patterson Larry L. Needham Sangil Lee Armistead G. Russell James A. Mulholland 《Atmospheric environment (Oxford, England : 1994)》2009,43(5):1043-1050
Twenty-four hour PM2.5 samples from a rural site, an urban site, and a suburban site (next to a major highway) in the metropolitan Atlanta area in December 2003 and June 2004 were analyzed for 19 polycyclic aromatic hydrocarbons (PAH). Extraction of the air samples was conducted using an accelerated solvent extraction method followed by isotope dilution gas chromatography/mass spectrometry determination. Distinct seasonal variations were observed in total PAH concentration (i.e. significantly higher concentrations in December than in June). Mean concentrations for total particulate PAHs in December were 3.16, 4.13, and 3.40 ng m?3 for the urban, suburban and rural sites, respectively, compared with 0.60, 0.74, and 0.24 ng m?3 in June. Overall, the suburban site, which is impacted by a nearby major highway, had higher PAH concentration than did the urban site. Total PAH concentrations were found to be well correlated with PM2.5, organic carbon (OC), and elemental carbon (EC) in both months (r2 = 0.36–0.78, p < 0.05), although the slopes from the two months were different. PAHs represented on average 0.006% of total PM2.5 mass and 0.017% of OC in June, compared with 0.033% of total PM2.5 and 0.14% of OC in December. Total PAH concentrations were also correlated with potassium ion (r2 = 0.39, p = 0.014) in December, but not in June, suggesting that in winter biomass burning can potentially be an important source for particulate PAH. Retene was found at a higher median air concentration at the rural site than at the urban and suburban sites—unlike the rest of the PAHs, which were found at lower levels at the rural site. Retene also had a larger seasonal difference and had the weakest correlation with the rest of the PAHs measured, suggesting that retene, in particular, might be associated with biomass burning. 相似文献
194.
Time perspective and climate change policy 总被引:1,自引:0,他引:1
The tendency to foreshorten time units as we peer further into the future provides an explanation for hyperbolic discounting at an inter-generational time scale. We study implications of hyperbolic discounting for climate change policy, when the probability of a climate-induced catastrophe depends on the stock of greenhouse gasses. We characterize the set of Markov perfect equilibria (MPE) of the inter-generational game amongst a succession of policymakers. Each policymaker reflects her generation's preferences, including its hyperbolic discounting. For a binary action game, we compare the MPE set to a “restricted commitment” benchmark. We compare the associated “constant-equivalent discount rates” and the willingness to pay to control climate change with assumptions and recommendations in the Stern Review on Climate Change.“…My picture of the world is drawn in perspective…. I apply my perspective not merely to space but also to time”—Ramsey. 相似文献
195.
Faulkner JW Zhang W Geohring LD Steenhuis TS 《Journal of environmental management》2011,92(3):587-595
Silage bunker runoff can be a very polluting substance and is increasingly being treated by vegetative treatment areas (VTAs), but little information exists regarding nutrient removal performance of systems receiving this wastewater. Nutrient transport through the shallow subsurface of three VTAs (i.e. one VTA at Farm WNY and two VTAs at Farm CNY) in glaciated soils containing a restrictive layer (i.e., fragipan) was assessed using a mass balance approach. At Farm WNY, the mass removal of ammonium was 63%, nitrate was 0%, and soluble reactive phosphorus (SRP) was 39%. At Farm CNY, the mass removal of ammonium was 79% in the West VTA, but nitrate and SRP increased by 200% and 533%, respectively. Mass removal of ammonium was 67% in the East VTA at Farm CNY; nitrate removal was 86% and SRP removal was 88%. The East VTA received a much higher nutrient loading, which was attributed to a malfunctioning low-flow collection apparatus within the settling basin. Results demonstrate that nutrient reduction mechanisms other than vegetative uptake can be significant within VTAs. Even though increases in nitrate mass were observed, concentrations in 1.65m deep wells indicated that groundwater impairment from leaching of nitrate was not likely. These results offer one of the first evaluations of VTAs treating silage bunker runoff, and highlight the importance of capturing concentrated low flows in VTA systems. 相似文献
196.
In 2008 the authors developed a methodology for assessing underground coal mines for high risk for major-hazard events. It focused on major hazard-related violations of safety standards associated with high-risk conditions. Later using the same stratified pilot sample of 31 mines, injury measures and MSHA citation data were combined into a Safe Performance Index (SPI). Using 2009 data, the database was expanded to 107 mines, which is a 30% sampling of all underground coal mines. The SPI was used to assess the relative safety-related risk of mines, including by mine-size category. The methodology can be used to assist companies, the Mine Safety and Health Administration, or state agencies in targeting mines with high risk for serious injuries and elevated citations for remediation of their violation and/or injury experience. 相似文献
197.
198.
Larry W. Mays 《Environmental Fluid Mechanics》2008,8(5-6):471-484
Hydraulic technology began during antiquity long before the great works of such investigators such as Leonardo da Vinci (1452–1519), Galileo Galilei (1564–1642), Evangelista Torricelli (1608–1647), Blaise Pascal (1623–1662), Isaac Newton (1642–1727), Daniel Bernoulli (1700–1782), and Leonhard Euler (1707–1783). The history of hydraulic technology even began long before Archimedes (287–212 BC). It is amazing to see what was accomplished in the application of hydraulic technology during antiquity, millenniums before the development of the concepts of conservation of mass, energy, and momentum used in present-day hydraulic design. The first developments of hydraulic technology occurred with the development of irrigation for agriculture, followed by the development of urban centers. A brief history is given of the development of hydraulic technology during antiquity. 相似文献
199.
John J. Gagliardi Don Kim Jennifer J. Sokol Larry A. Zazzera Vincent D. Romero Matthew R. Atkinson Faisal Nabulsi Harry Zhang 《Journal of Manufacturing Processes》2013,15(3):348-354
An alternative, and potentially better, means to process prime LED sapphire wafers is proposed. A brief history of material removal mechanisms suggests that brittle materials can be abraded in a ductile regime and that this can be a superior mode of removal. Advantages in material removal rate, surface finish and subsurface damage for a 2-body ductile removal of sapphire are shown relative to 3-body material removal. A major difficulty with this approach may be getting the LED manufacturing industry to understand that the appearance of the sapphire surface is quite different, unfamiliar yet predictable. Finally, some potential alternative process approaches are discussed that utilize 2-body material removal. 相似文献
200.